Updating the Slicer extension is not trivial and thus it always takes us quite a bit to update them when new versions of 3D slicer are released. 

You could either use the version that is in the download section here (which is the 2017 version, so I am not 100% sure it works with Catalina) or download an older version of Slicer that has a working extension.

as a note, we are working on a significantly changed new version that will also provide susceptibility correction (via FSL) .

Martin